EP1617437B1

EP1617437B1 - Phase change memory device and programming and controlling methods

Info

Publication number: EP1617437B1
Application number: EP05014605A
Authority: EP
Inventors: Baek-Hyung Cho; Du-Eung Jinsan Maeul Samsung Apt. 516-1004 Kim; Woo-Yeong Salgugol Jindeok Apt. 703-703 Cho
Original assignee: Samsung Electronics Co Ltd
Current assignee: Samsung Electronics Co Ltd
Priority date: 2004-07-09
Filing date: 2005-07-06
Publication date: 2013-03-20
Anticipated expiration: 2025-07-06
Also published as: US7304885B2; TWI371753B; KR20060004289A; TW200603154A; US20100097850A1; KR100587702B1; US20110242886A1; US7643335B2; EP1617437A1; US20080137402A1; US7944741B2; CN1734671B; JP2006024355A; US20060007729A1; JP4832817B2; CN1734671A; US8194442B2

Description

The invention relates to a phase change memory device as well as a method of programming such memory device and a method of controlling write driver circuits used therein. This application is related to and claims priority from Korean Patent Application No. 204-0053346 .
A phase change memory element is a memory element for storing information using the characteristics of electric conductivity or a resistance difference between a crystalline phase and an amorphous phase of a specific phase change material. The phase change memory element forms a memory cell electrically connected to a transistor element or the like, formed on a semiconductor substrate for addressing and read/write operations of the device. In the memory element, information is stored using a conductivity difference in accordance with the phase change of a region of a memory layer.
FIG. 1A and FIG 1B illustrate a conventional phase change memory cell 10. As seen in FIG. 1A , the phase change memory cell 10 includes a phase change material 14 between a top electrode 12 and a bottom electrode 18. To increase the current density and, thereby, improve the efficiency of heating of the phase change material 14, the bottom electrode 18 may be connected to the phase change material 14 through a bottom electrode contact (BEC) 16 that has reduced surface area in comparison to the bottom electrode 18. An access transistor 20 may be connected to the bottom electrode 18 and controlled by a word line.
As seen in FIGs. 1A , 1B and 2 , the phase change memory cell 10 operates such that a current flowing through the phase change material 14 electrically heats a phase change region, and the structure of the phase change material 14 is reversibly changed to a crystalline state ( FIG. 1A ) or an amorphous state ( FIG. 1B ) to store information. In FIG. 1B, the region of the phase change material 14 that changes state to an amorphous state is illustrated by the cross-hatched region adjacent the BEC 16. The stored information can be read by flowing a relatively low current through the phase change region and measuring the resistance of the phase change material. Thus, FIG. 2 illustrates a conventional phase change memory cell 10 where a cell transistor 20 is controlled by a word line WL to control the flow of current ICELL from a bit line BL through the variable resistance C provided by the phase change material.
In setting the region of the phase change material 14 to an amorphous state or a crystalline state, different pulses may be used to control the heating of the phase change material 14. As seen in FIG. 3 , a high temperature short duration heating cycle 35 is used to reset the phase change material 14 to an amorphous state and a longer duration lower temperature heating cycle 36 is used to set the phase change material 14 to a crystalline state. In particular, in the short duration cycle 35, the phase change material 14 is heated to a temperature above the melting point, Tm, of the phase change material 14 and then quickly cooled, e.g., within a few nanoseconds, to create an amorphous region in the phase change material 14. In the longer duration cycle 36, the phase change material 14 is heated to a temperature above a crystallizing point, Tx, and below the melting point, Tm, of the phase change material 14 and maintained at that temperature for a predetermined time before cooling to create a crystallized region in the phase change material 14. Thus, the temperature is maintained within a set window of above the crystallizing temperature Tx and below the melting temperature Tm.
FIG. 4 illustrates various current waveforms for programming phase-change memories. In particular, as seen in FIG. 4 , the reset current is of shorter duration but greater amplitude than the set current. If multiple memory cells (e.g., more than 16 bits) are reset simultaneously, the peak current may exceed the capability of the power supply, which may result in fluctuations in the output of the power supply. Typically, the number of memory cells in a block of memory cells that are simultaneously programmed (set and reset) has been limited by the reset current considerations.
Various techniques for programming phase-change memory cells are described, for example, in Patent publications US 6,545,907 ; US 6,075,719 ; and US 6,487,113 .
US 6,480,438 B1 discloses a phase-change memory device comprising bit line and word line compensation elements to reduce differences in resistances along bit lines and word lines to which the cells are coupled in a matrix configuration with driver circuitry at one side of the word line rows and bit line columns, respectively. For programming the cells sequential individual program voltage pulses are applied to the word lines by the corresponding word line driver circuitry.
It is the technical problem underlying the invention to provide a phase change memory device as well as a method of programming such device, which are capable to at least partly overcome the above-mentioned difficulties of the prior art, and in particular to avoid an undesirably high peak current in reset operations preferably without substantially increasing the programming time for writing data.
The invention solves this problem by providing a phase change memory device having the features of claim 1 and a method of programming a memory device having the features of claim 18.
Advantageous embodiments of the invention are mentioned in the subclaims, the wording of which is herewith incorporated into the specification by reference to avoid unnecessary text repetition.
The invention also provides a method of controlling write driver circuits of a phase-change memory device by providing a different, non-overlapping, reset control signal to each of a plurality of write driver circuits of the phase-change memory device. A same set control signal may be provided to each of the plurality of write driver circuits. In certain embodiments, a sum of durations of the different reset control signals is not greater than a duration of the set control signal.
Advantageous embodiments of the invention are described in detail below with reference to the attached drawings in which also the conventional embodiments are shown as explained above to facilitate the understanding of the invention. In the drawings:

FIGs. 1A and 1B are illustrations of a conventional phase change memory cell.
FIG. 2 is a schematic diagram of a conventional phase change memory cell.
FIG. 3 is graph illustrating the change in state of a phase change material used for conventional phase change memory cells as a function of time and temperature.
FIG. 4 is a graph illustrating different set and reset pulses for conventional phase change memories.
FIG. 5 is a block diagram of a portion of a phase change memory according to the invention.
FIG. 6 is a timing diagram illustrating the timing of set and reset signals of the phase change memory of FIG. 5 according to the invention.
FIG. 7 is a block diagram of a write driver circuit according to the invention.
FIG. 8 is a schematic circuit diagram of the write driver of FIG. 7 according to the invention.
FIG. 9 is a circuit diagram of a reset control signal generator according to the invention.
FIG. 10 is a block diagram of a portion of a further phase change memory according to the invention.
FIG. 11 is a timing diagram illustrating the timing of set and reset signals of the phase change memory of FIG. 10 according to the invention.

The invention now will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. In the drawings, the thickness of layers and regions may be exaggerated for clarity. Like numbers refer to like elements throughout.
As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.
It will be understood that when an element such as a layer, region or substrate is referred to as being "on" or extending "onto" another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being "directly on" or extending "directly onto" another element, there are no intervening elements present. It will also be understood that when an element is referred to as being "connected" or "coupled" to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being "directly connected" or "directly coupled" to another element, there are no intervening elements present.
Embodiments of the invention provide for sequentially providing reset pulses to phase change memory cells so as to reduce the peak current required to program the memory cells from that required if the memory cells are reset simultaneously.
FIG. 5 is a schematic diagram of a phase change memory device 100 configured to provide sequential reset pulses according to the invention. As seen in FIG. 5 , the phase change memory device 100 includes a plurality of phase change memory cells MC that include an access transistor AT and a variable resistance that includes a phase change material GST. The access transistors AT are connected to respective word lines WL0 through WLm to control turning the access transistors AT on and off. The memory cells are also connected to respective bit lines BLi_0...BLi_n, BLj_0...BLj_n and BLk_0...BLk_n. Respective select transistors ST1, ST2, ST3, ST4, ST5 and ST6 are also provided to selectively couple the bit lines to a corresponding one of a plurality of write driver circuits 120, 122 and 124.
The word lines may be responsive to a row address decoder (not shown) and the bit lines may be selected responsive to a column address decoder (not shown). Such row and address decoders are well known in the art and need not be described further herein. As used herein, a word of data refers to the number of bits that are selected for a given output of the row address decoder and column address decoder (i.e. when a word line is active and a corresponding column select signal is active). In some embodiments of the invention, a word includes 16 bits of data.
Furthermore, as illustrated in FIG. 5 , a write driver may be coupled to multiple bit lines. The multiple bit lines may be from different words of data. In other embodiments, the some or all of the bit lines coupled to a write driver may be from the same word of data. As illustrated in FIG. 5 , however, the bit lines coupled to the respective write drivers 120, 122 and 124 are from different words of data. Thus, in the embodiments illustrated in FIG. 5 , the bit lines BLi_0 through BLi_n represent the ith bit of n words of data. In some embodiments of the present invention, four bit lines are coupled to each of the write drivers 120, 122 and 124.
As is further illustrated in FIG. 5 , the plurality of write drivers 120, 122 and 124 each receive a common set signal and individual reset signals RESET i, RESET j and RESET k. As is illustrated in FIG. 6 , the set signal Set may be a signal with a relatively long duration and the respective reset signals RESET i, RESET j and RESET k each have a shorter duration and are sequenced in time. The reset signals RESET i, RESET j and RESET k are provided as sequential reset pulses by a reset signal generator. Examples of a reset signal generator are discussed in further detail below. Thus, the reset signals RESET i, RESET j and RESET k may be sequentially applied to the write drivers 120, 122 and 124 such that fewer than all of the write drivers for a word of data are simultaneously active, which may reduce the peak current requirements when the memory cells are reset.
In some embodiments of the invention, the set signal Set has a duration of from about 100 to about 500 nanoseconds (ns) and the sequential reset signals RESET i, RESET j and RESET k each have a duration of from about 10 to about 50 ns. The sequential reset signals are non-overlapping and, in some embodiments, may be spaced apart from each other by about 10 ns. In the embodiments of the invention, the sum of the durations of the sequential reset signals is less than the duration of the set signal. By making the sum of the durations of the reset signals not greater than the duration of the set signal, the peak reset current may be reduced without extending the time it takes to program a word of data of the phase change memory device. Furthermore, while three reset signals are illustrated in FIGs. 5 and 6 , fewer or greater numbers of reset signals may be provided. In the embodiments of the present invention, the number of reset signals is limited to the number of reset signals that may be provided without having the sum of the durations of the reset signals exceed the duration of the set signal.
While the embodiments illustrated in FIG. 5 show three write drivers 120, 122 and 124, such an illustration is merely an example configuration of a portion of a phase change memory device and fewer or greater numbers of write drivers may be provided. Furthermore, while FIG. 5 illustrates individual write drivers each having a separate reset signal, according to some embodiments of the invention, groups of write drivers may receive the same signal, so long as the groups contain fewer than all of the write drivers for a word of data. Thus, for example, two or more write drivers in the same word of data may each receive the same reset signal. The groups of write drivers that receive the same reset signal may be uniform in number or non-uniform. For example, in some embodiments of the invention, four sequential reset signals may be provided, each to four write drivers such that a 16 bit word of data is divided across four groups of write drivers.
FIG. 7 is a diagram of a write driver circuit according to the invention that may be suitable for use as the write drivers 120, 122 and/or 124. As seen in FIG. 7 , a write driver circuit according to some embodiments of the invention include a current control circuit 200, a current driver circuit 202 and a pulse selection circuit 204. A bias voltage DC_BIAS is provided to the current control circuit 200. The current control circuit 200 is responsive to the pulse selection circuit 204 and controls the current driver 202 to output either a set or a reset pulse. The pulse selection circuit 204 receives a set pulse and one of the sequential reset pulses as illustrated in FIG. 6 and the data to be programmed in the phase change memory cell and uses the data to select one of the set pulse and the sequential reset pulse. The selected one of the set pulse and the sequential reset pulse controls the current control circuit and the current driver to drive the bit line at the time and for the duration substantially corresponding to the timing of the selected one of the set pulse and the sequential reset pulse.
FIG. 8 is a more detailed schematic diagram of the write driver circuit of FIG. 7 according to some embodiments of the invention. As seen in FIG. 8 , the DATA input selects between the SET and RESETi inputs and provides a signal corresponding to the selected one of the SET and RESETi inputs to the current control circuit 200. The current control circuit 200 controls the current supplied by the current driver circuit 202 by controlling the drive transistor PM3.
The duration of the output pulse I_SET/I_RESETi is controlled by the output of the inverter IN4 such that, when the output of the inverter IN4 is at a high level, the transistors NM6 and PM2 are off and the control of the drive transistor PM3 is based on the voltage of the node ND1 of the current control circuit 200. When the output of the inverter IN4 is at a low level, the transistor PM2 turns on, which turns off the drive transistor PM3. When the output of the inverter IN4 is at a low level, the output of the inverter IN5 is at a high level, which turns on the transistor NM6 and terminates the set/reset pulse I_SET/I_RESETi being at a high level. The output of the inverter IN4 is controlled by the selected one of the SET and RESETi inputs through the inverter IN3. Thus, the duration of the set/reset pulse I-SET/I_RESETi applied to a bit line by the write driver circuit may be controlled by the pulse duration of the selected one of the SET and RESETi inputs.
When the DATA input is at a high level so as to select the RESETi input through the pass gate PG2, the output of the inverter IN1 is low and the output of the inverter IN2 is high. Thus, the transistors NM3 and NM4 are turned on. The transistors NM1 and NM2 are always on as a result of being controlled by the DC_BIAS input. When the RESETi signal is at a high level, the output of the inverter IN4 is at a high level which turns on the transistor NM5 and turns off the transistor PM2. Currents of i1 flowing through the transistors NM1 and NM2 and i2 flowing through the transistors NM3 and NM4 are mirrored, so that a reset current of i1+i2 is provided at the output node ND2. When the reset signal RESETi returns to a low level, the transistors PM3 is turned off and the transistor NM6 is turned on to terminate the reset pulse as described above.
When the DATA input is at a low level so as to select the SET input through the pass gate PG1, the output of the inverter IN1 is high and the output of the inverter IN2 is low. Thus, the transistors NM3 and NM4 are turned off. The transistors NM1 and NM2 are always on as a result of being controlled by the DC_BIAS input. When the SET signal is at a high level, the output of the inverter IN4 is at a high level which turns on the transistor NM5 and turns off the transistor PM2. Only the current i1 flowing through the transistors NM1 and NM2 is mirrored, so that a set current of i1 is provided at the output node ND2. When the set signal SET returns to a low level, the transistors PM3 is turned off and the transistor NM6 is turned on to terminate the set pulse as described above.
FIG. 9 is a schematic diagram of a reset signal generation circuit according to the invention. As seen in FIG. 9 , the reset pulse may be generated responsive to an address transition detection signal (ATD). In other embodiments of the present invention, the reset pulse may be generated responsive to a data transition detection signal (DTD). The ATD signal is provided to a NOR gate NOR1 that acts as an inverter such that when the ATD signal is high, the output of the of the NOR gate NOR1 is low and when the ATD signal is low, the output of the of the NOR gate NOR1 is high. The output of the NOR gate NOR1 is coupled to the input of a NAND gate NAN1 and to a delay element D1. The delay element D1 may be used to control the duration of the sequential reset pulses. Thus, the delay element D1, in some embodiments of the invention, provides a pulse of from about 10 to about 50 ns. The output of the NAND gate NAN1 is provided to a plurality of serially connected delay elements D2, D3 and D4 where the output of the delay elements is provided as a subsequent one of the plurality of sequential reset pulses. In some embodiments of the invention, the delay elements D2, D3 and D4 have a delay that is greater than the duration of the pulse output by the NAND gate NAN1 so as to provide non-overlapping sequential pulses. In some embodiments, the delay of the delay elements D2, D3 and D4 is greater than the duration of the pulse output by the NAND gate NAN1 by about 10 ns.
FIG. 10 is a schematic diagram of a portion of a phase change memory device 300 according to further embodiments of the invention. As seen in FIG. 10 , the phase change memory cells may be provided as described above with reference to FIG. 5 . However, the write driver circuits 320, 322, 324 and 326 are each connected to a single bit line. In such a case, the reset RESET1... RESETn and set Set signals may be provided as illustrated in FIG. 11 . The write driver circuits 320, 322, 324 and 326 may be provided as described above with reference to FIGs. 7 and 8 . Furthermore, the reset pulse generation circuit of FIG. 9 may be utilized to provide sequential reset pulses to each of the individually connected write driver circuits 320, 322, 324 and 326. The durations and relationship of the reset pulses and the set pulse may be as described above with reference to FIGs. 5 and 6 .
In operation, the phase change memory device 300 of FIG. 10 may provide sequential reset pulses to the respective write driver circuits 320, 322, 324 and 326. Because the reset pulses are sequentially applied to the write driver circuits 320, 322, 324 and 326, the peak current required for resetting phase change memory cells may be reduced as the number of write driver circuits 320, 322, 324 and 326 that are simultaneously driving the bit lines may be reduced.
As described above, the invention provides phase-change memory devices that include a plurality of phase-change memory cells and means for sequentially applying a reset pulse to subsets of the phase-change memory cells that are commonly connected to a word line. The means for sequentially applying a reset pulse may be provided, for example by the write driver circuits 120, 122 and 124 and/or 320, 322, 324 and 326, pulse generation circuits of FIG. 9 and interconnection of the write driver circuits to the phase change memory cells as described above with reference to FIGs. 5 and/or 10. In particular embodiments of the invention, the plurality of phase-change memory cells are reset by applying a signal of a first pulse width to the phase-change memory cells and set by applying a signal of a second pulse width to the phase-change memory cells. In such a case, the means for sequentially applying a reset pulse may comprise means for sequentially applying a reset pulse wherein a duration of each of the sequentially applied reset pulses corresponds to the first pulse width and a sum of the durations of the sequentially applied reset pulses is not substantially greater than the second pulse width. Thus, the peak current may be reduced without increasing the programming timing for writing a word of data. Such means for sequentially applying a reset pulse may be provided by the configuration of the delay lines or other such pulse generation circuits that provided the appropriate timing relationships.
Furthermore, some embodiments of the invention provide means for applying a common set pulse to the subsets of the phase-change memory cells that are commonly connected to a word line. Such means may be provided, for example, by the interconnection of a set pulse generation circuit to the write driver circuits as discussed above.
While embodiments of the invention have been described with reference to particular exemplary write driver circuits and/or reset pulse generation circuits, other techniques and/or circuits for providing sequentially applied reset pulses may be utilized. For example, pulse generation circuits that do not use delay lines may be utilized or any circuit that provides the desired sequential pulses with the appropriate timing relationship. Furthermore, the pulse width and sequential application of reset pulses to the bit lines could be accomplished outside the write driver circuits, for example, by serially activating the access transistors ST1, ST3 and ST5 while providing a common reset signal to the write driver circuits and controlling the reset pulse duration by controlling the duration of time that the access transistors are active.
Embodiments of the invention have been described with reference to particular divisions of functions between, for example, a write driver circuit and a pulse generation circuit or functions within the write driver circuits. However, blocks in the diagrams may be combined or otherwise rearranged while still falling within the scope of the invention. Accordingly, the invention is not limited to the particular illustrative examples discussed above but may include any circuit capable of carrying out the functions or operations described herein for sequentially providing reset pulses to phase change memory cells.

Claims

A phase-change memory device comprising:
a plurality of phase-change memory cells (MC) each adapted to change state between a set state and a reset state, and

write driver means (120, 122, 124) coupled to the phase change memory cells,
characterized in that
the write driver means (120, 122, 124) are adapted to sequentially apply individual reset pulses to corresponding subsets of the phase-change memory cells that are commonly connected to a word line (WL0 to WLm) to drive them to their reset state, and to apply a common set pulse to these subsets of phase-change memory cells to drive them to their set state, wherein fewer than all of the write drivers means (120, 122, 124) for a word of data are simultaneously active and a sum of the durations of the reset pulses is not greater than the duration of the set pulse.
The phase-change memory device of Claim 1, wherein the plurality of phase-change memory cells are reset by applying the sequential reset pulses with a first pulse width to the phase-change memory cells and set by applying the common set pulse with a second pulse width to the phase-change memory cells.
The phase-change memory device of Claim 1 or 2, wherein the subsets of the phase-change memory cells are configured so that a different reset pulse is applied to each individual of a plurality of bit lines (BLi_0 to BLK_n) of the memory device coupled to the phase-change memory cells or so that a same reset pulse is applied to at least two different bit lines (BLi_0 to BLin) of the memory device.
The phase-change memory device of Claim 3, wherein the at least two different bit lines are each from different words of the memory device.
The phase-change memory device of any of Claims 1 to 4, wherein the write driver means comprise a plurality of write driver circuits (120, 122, 124) each coupled to corresponding ones of the phase change memory cells wherein each of the plurality of write driver circuits receives a different, non-overlapping reset control signal.
The phase-change memory device of Claim 5, wherein each of the write driver circuits also receives a same set control signal.
The phase-change memory device of Claim 6, wherein a sum of durations of the different reset control signals is not greater than a duration of the set control signal.
The phase-change memory device of any of Claims 1 to 7, further comprising a reset pulse generation circuit configured to output the plurality of sequential reset pulses, each sequential reset pulse being output to a corresponding one of a plurality of reset lines, wherein the plurality of write driver circuits (120, 122, 124) are coupled to respective ones of the reset lines of the reset pulse generation circuit.
The phase-change memory device of any of Claims 5 to 8, wherein the plurality of write driver circuits are coupled to corresponding phase change memory cells corresponding to a word of data.
The phase-change memory device of Claim 8 or 9, wherein the reset pulse generation circuit comprises:
a first pulse generation circuit (NOR1, D1, NAN1) configured to generate a first reset pulse; and

a plurality of delay lines (D2, D3, D4) serially coupled to the first pulse generation circuit to provide successive sequentially delayed reset pulses.
The phase-change memory device of Claim 10, wherein the first pulse generation circuit is responsive to an address transition detection signal (ATD) or to a data transition detection signal to generate the first reset pulse.
The phase-change memory device of Claim 10 or 11, wherein the first pulse generation circuit comprises a NAND logic gate having a first control signal as a first input and a delayed version of the first control signal as a second input.
The phase-change memory device of Claim 12, wherein the first pulse generation circuit further comprises a delay line that receives as an input the first control signal and outputs the delayed version of the first control signal to the second input of the NAND logic gate.
The phase-change memory device of any of Claims 10 to 13, wherein a delay of each of the delay lines is greater than a pulse width of the first reset pulse.
The phase-change memory device of any of Claims 5 to 14, wherein the plurality of write driver circuits are each coupled to a single bit line or a plurality of bit lines of the phase-change memory cells.
The phase-change memory device of any of Claims 1 to 15, wherein the common set pulse of the phase-change memory device has a duration of from about 100 ns to about 500 ns and each of the sequential reset pulses has a duration of from about 10 ns to about 50 ns.
The phase-change memory device of any of Claims 1 to 16, wherein the sequential reset pulses are spaced apart from each other by about 10 ns.
A method of programming a memory device comprising a plurality of phase-change memory cells each being adapted to change state between a set state and a reset state,
characterized by
sequentially applying individual reset pulses corresponding to subsets of the phase-change memory cells that are commonly connected to a word line (WL0 to WLm) to drive them to their reset state, and applying a common set pulse to these subsets of phase-change memory cells to drive them to their set state, wherein fewer than all of the write drivers means (120, 122, 124) for a word of data are simultaneously active and a sum of the durations of the reset pulses is not greater than the duration of the set pulse.
The method of Claim 18, wherein the plurality of phase-change memory cells are reset by applying the reset pulses with a first pulse width to the phase-change memory cells and set by applying the set pulse with a second pulse width to the phase-change memory cells.
The method of Claim 18 or 19, wherein the memory programmed is the phase-change memory device of any of Claims 1 to 17.
The method of any of Claims 18 to 20, wherein the sequentially applied reset pulses are not overlapping.
The method of any of Claims 18 to 21, wherein the sequentially applied reset pulses are generated by respective write drivers responsive to corresponding reset control signals and wherein each of the write drivers receives a same set control signal.
The method of Claim 22, further comprising providing a same set control signal to each of the plurality of write driver circuits.